An anisotropically conductive sheet is a sheet exhibiting conductivity only in its thickness-wise direction or having pressure-sensitive conductive conductor parts exhibiting conductivity only in the thickness-wise direction when they are pressed in the thickness-wise direction. As such anisotropically conductive sheets, there have been known those of various structures, such as those obtained by uniformly dispersing metal particles in an elastomer (see, for example, Patent Art. 1), those obtained by unevenly dispersing a conductive magnetic metal in an elastomer, thereby forming a great number of conductive parts each extending in a thickness-wise direction thereof and an insulating part for mutually insulating them (see, for example, Patent Art. 2) and those obtained by defining a difference in level between the surface of each conductive part and an insulating part (see, for example, Patent Art. 3).
In these anisotropically conductive sheets, conductive particles are contained in an insulating elastic polymeric substance in a state oriented so as to align in the thickness-wise direction, and each conductive path is formed by a chain of a great number of conductive particles.
Since such an anisotropically conductive sheet has such features that compact electrical connection can be achieved without using any means such as soldering or mechanical fitting, and that soft connection becomes feasible with mechanical shock or strain absorbed therein, it is widely used as an anisotropically conductive connector for achieving electrical connection between circuit devices, for example, electrical connection between a printed circuit board and a leadless chip carrier, liquid crystal panel or the like, in fields of, for example, electronic computers, electronic digital clocks, electronic cameras and computer key boards.
Further, as a means for achieving electrical connection between electrodes to be inspected of a circuit device, which is an object of inspection, and electrodes for inspection formed on a front surface of a circuit board for inspection in electrical inspection of circuit devices such as printed circuit boards and semiconductor integrated circuits, an anisotropically conductive sheet is used in place of a probe member, in which a plurality of pin probes are arranged corresponding to the electrode to be inspected.
However, these anisotropically conductive sheets have involved problems that when conductive parts each forming a conductive path are compressed by about 20% or more of the thickness thereof to be deformed, conductive particles adjoining one another in a horizontal direction come into contact with one another, so that conductivity is produced in not only the thickness-wise direction but also the horizontal direction to impair the anisotropic conductivity, and that the elastic polymeric substance forming the conductive parts cause permanent set to deform the conductive parts and shorten the durability of the anisotropically conductive sheets. The anisotropically conductive sheets have also involved a problem that when the thickness of the conductive parts is made great, conductive particles are oriented in not only the thickness-wise direction but also a direction of an adjoining conductive part when such an anisotropically conductive sheet is formed by applying a magnetic field, and so an insulation resistance value between the conductive parts adjoining each other is lowered to fail to achieve good electrical properties.
Therefore, the conventional anisotropically conductive sheets have involved a problem that since the anisotropically conductive sheet is pressurized by a high pressure when circuit boards wide in a scatter of height are connected, the durability upon continuous use of such an anisotropically conductive sheet becomes low.
An anisotropically conductive sheet having a large area and a great number of conductive parts is required for connection of circuit boards of, for example, a wafer inspection apparatus. When such an anisotropically conductive sheet having a large area and a great number of conductive parts is produced, however, it is difficult to provide a sheet, all the conductive parts of which have electrical properties within a certain range, so that a problem of low productivity has arisen. A problem of high production cost has also arisen because large-sized mold and magnetic field molding machine are required.
In a wafer inspection apparatus for inspecting a wafer having a diameter of, for example, 12 inches, an anisotropically conductive sheet is used for electrically connecting circuit boards for inspection to each other. This anisotropically conductive sheet is required to have a diameter of about 12 inches or more. However, when such an anisotropically conductive sheet is pinched by the circuit boards for inspection, conductive parts located at a peripheral region thereof are sufficiently compressed, whereas conductive parts located at a central region thereof are not sufficiently compressed, so that a problem that conductivity of the conductive parts becomes uneven has arisen.
When the anisotropically conductive sheet has been used repeatedly many times, the conductivity of the conductive parts located at the peripheral region, to which higher pressurizing force is applied, is lowered, so that a problem that a long working life is not achieved has arisen.
The anisotropically conductive sheet having a large area and a great number of conductive parts, which is used for connection of the circuit boards of the wafer inspection apparatus is high in production cost. When the conductivity of the conductive parts located at the peripheral region of the anisotropically conductive sheet is lowered, and the use of their conductive parts becomes difficult when inspection of wafers is made repeatedly many times, the anisotropically conductive sheet itself must be exchanged for a new anisotropically conductive sheet, so that a problem that inspection cost of inspection of the wafers increases has arisen.
Patent Art. 1: Japanese Patent Application Laid-Open No. 93393/1976;
Patent Art. 2: Japanese Patent Application Laid-Open No. 147772/1978;
Patent Art. 3: Japanese Patent Application Laid-Open No. 250906/1986.